Carbon dioxide gas (CO2) may be converted into liquid fuels such as, for example, hydrocarbon molecules of between about 5 and about 12 carbon atoms per molecule (e.g., gasoline) through multi-step reactions. For example, carbon dioxide (CO2) gas and hydrogen (H2) may be converted to carbon monoxide (CO) gas and water (H2O) through the Reverse Water-Gas Shift Reaction, which is shown by Reaction [1] below.CO2+H2→CO+H2O  [1]
Synthesis gas, which is a mixture of carbon monoxide gas (CO) and hydrogen gas (H2) then may be produced from the reaction products of the Reverse Water-Gas Shift Reaction by adding additional hydrogen gas (H2) to the reaction products. This synthesis gas may be further reacted through either Fischer-Tropsch (FT) processes, or through methanol synthesis (MS) plus methanol-to-gasoline (MTG) processes, to provide liquid fuels.
Briefly, Fischer-Tropsch processes include various catalyzed chemical reactions in which synthesis gas is converted into liquid hydrocarbons in a reactor in the presence of a catalyst and at temperatures between about 200° C. and about 350° C. Catalysts used in Fischer-Tropsch processes include, for example, iron, cobalt, nickel, and ruthenium. While various interrelated reactions may occur in Fischer-Tropsch processes, the overall reaction process may be generally represented by Reaction [2] below.(2n+1)H2+nCO→CnH2n+2+nH2O  [2]
As mentioned above, synthesis gas may also be reacted by first performing a methanol synthesis (MS) process, and then performing a methanol-to-gasoline (MTG) process to produce liquid fuels. Methanol synthesis (MS) processes involve the catalytic conversion of carbon monoxide, carbon dioxide, hydrogen, and water to methanol and other reaction byproducts. The methanol synthesis reactions may be generally represented by Reactions [3], [4], and [5] below.CO+2H2→CH3OH  [3]CO2+3H2→CH3OH+H2O  [4]CO+H2O→CO2+H2  [5]
The methanol-to-gas (MTG) process involves the conversion of methanol to hydrocarbon molecules using zeolite catalysts, which are described in further detail below. The methanol-to-gas (MTG) process occurs in two steps. First, methanol is heated to about 300° C. and partially dehydrated over an alumina catalyst at about 2.7 megapascals to yield an equilibrium mixture of methanol, dimethyl ether, and water. This effluent is then mixed with synthesis gas and introduced into a reactor containing a zeolite catalyst (such as, for example, a ZSM-5 zeolite), at temperatures between about 350° C. and about 366° C. and at pressures between about 1.9 megapascals and about 2.3 megapascals, to produce hydrocarbons and water. The methanol-to-gas (MTG) reactions may be generally represented by Reactions [6], [7], and [8] below.2CH3OH→CH3OCH3+H2O  [6]CH3OCH3→C2-C5 Olefins  [7]C2-C5 Olefins→Paraffins, Cycloparaffins, Aromatics  [8]
While the feasibility of the above-described reactions has been demonstrated, mass production of liquid fuels using such processes has not been widely implemented due, at least in part, to the relatively high costs associated with carrying out the reactions, and to the relatively low yields exhibited by the reactions.
In an effort to improve the yield of the various reactions and to minimize the costs associated with carrying out the reactions, research has been conducted in an effort to improve the efficiency of the catalysts associated with each of the respective catalyzed reactions. As previously mentioned, zeolites have been used as catalysts in the methanol-to-gas (MTG) process.
Zeolites are substantially crystalline oxide materials in which the crystal structure of the oxide material defines pores, channels, or both pores and channels in the oxide material. Such pores and channels may have cross-sectional dimensions of between about 1 angstrom and about 200 angstroms, and typically have cross-sectional dimensions of between about 3 angstroms and about 15 angstroms. Typically, zeolite materials include metal atoms (classically, silicon or aluminum) that are surrounded by four oxygen anions to form an approximate tetrahedron consisting of a metal cation at the center of the tetrahedron and oxygen anions at the four apexes of the tetrahedron. The tetrahedral metals are often referred to as “T-atoms.” These tetrahedra then stack in substantially regular arrays to form channels. There are various ways in which the tetrahedra may be stacked, and the resulting “frameworks” have been documented and categorized in, for example, Ch. Baerlocher, W. M. Meier and D. H. Olson, Atlas of Zeolite Framework Types, 5th ed., Elsevier: Amsterdam, 2001, the contents of which are hereby incorporated herein in their entirety by this reference.
Silicon-based tetrahedra in zeolitic materials are electrically neutral since silicon typically exhibits a 4+ oxidation state. Tetrahedra based on elements other than silicon, however, may not be electrically neutral, and charge-compensating ions may be present so as to electrically neutralize the non-neutral tetrahedra. For example, many zeolites are aluminosilicates. Aluminum typically exists in the 3+ oxidation state, and charge-compensating cations typically populate the pores to maintain electrical neutrality. These charge-compensating cations may participate in ion-exchange processes. When the charge-compensating cations are protons, the zeolite may be a relatively strong solid acid. The acidic properties of such solid acid zeolites may contribute to their catalytic properties. Other types of reactive metal cations may also populate the pores to form catalytic materials with unique properties.
Notwithstanding the research that has been conducted with respect to the above-described reactions and their respective catalytic materials, there remains a need in the art for catalytic materials and structures than can be used to provide a direct route or mechanism for the reduction of carbon monoxide (CO) and/or carbon dioxide (CO2) to liquid fuels. As used herein, the term “carbon oxide” (COx) means and includes at least one of CO and CO2.